CLINICAL PHARMACOLOGY
Cefaclor is well-absorbed after oral administration to
fasting subjects. Total absorption is the same whether the drug is given with
or without food; however, when it is taken with food, the peak concentration
achieved is 50% to 75% of that observed when the drug is administered to
fasting subjects and generally appears from three-fourths to 1 hour later.
Following administration of 250 mg, 500 mg, and 1 g doses to fasting subjects,
average peak serum levels of approximately 7, 13, and 23 mcg/mL, respectively,
were obtained within 30 to 60 minutes. Approximately 60% to 85% of the drug is
excreted unchanged in the urine within 8 hours, the greater portion being
excreted within the first 2 hours. During this 8-hour period, peak urine
concentrations following the 250 mg, 500 mg and 1 g doses were approximately
600, 900 and 1,900 mcg/mL, respectively. The serum half-life in normal subjects
is 0.6 to 0.9 hour. In patients with reduced renal function, the serum
half-life of cefaclor is slightly prolonged. In those with complete absence of
renal function, the plasma half-life of the intact molecule is 2.3 to 2.8
hours. Excretion pathways in patients with markedly impaired renal function
have not been determined. Hemodialysis shortens the half-life by 25% to 30%.
Microbiology
Mechanism Of Action
As with other cephalosporins, the bactericidal action of
cefaclor results from inhibition of cell-wall synthesis.
Mechanism Of Resistance
Resistance to cefaclor is primarily through hydrolysis of
beta-lactamases, alteration of penicillin-binding proteins (PBPs) and decreased
permeability. Pseudomonas spp., Acinetobacter calcoaceticus and
most strains of Enterococci (Enterococcus faecalis, group D
streptococci), Enterobacter spp., indole-positive Proteus,
Morganella morganii (formerly Proteus morganii), Providencia rettgeri
(formerly Proteus rettgeri), and Serratia spp. are resistant to
cefaclor. Cefaclor is inactive against methicillin-resistant staphylococci.
β-lactamase-negative, ampicillin-resistant strains of H. influenzae
should be considered resistant to cefaclor despite apparent in vitro to this
agent.
Antibacterial Activity
Cefaclor has been shown to be active against most strains
of the following microorganisms both in vitro and in clinical infections as
described in the INDICATIONS AND USAGE section.
Gram-positive Bacteria
Staphylococcus aureus (methicillin susceptible only)
Coagulase negative staphylococci (methicillin
susceptible only)
Streptococcus pneumoniae
Streptococcus pyogenes (group A β-hemolytic
streptococci)
Gram-negative Bacteria
Escherichia coli
Haemophilus influenzae (excluding
β-lactamase-negative, ampicillin-resistant strains)
Klebsiella spp.
Proteus mirabilis
The following in vitro data are available, but their
clinical significance is unknown. At least 90 percent of the
following bacteria exhibit an in vitro minimum inhibitory concentrations (MICs)
less than or equal to the susceptible breakpoint of cefaclor. However, the
safety and effectiveness of cefaclor in treating clinical infections due to
these bacteria has not been established in adequate and well-controlled trials.
Gram-negative Bacteria
Citrobacter diversus
Moraxella catarrhalis
Neisseria gonorrhoeae
Anaerobic Bacteria
Bacteroides spp.
Peptococcus spp.
Peptostreptococcus spp.
Propionibacterium acnes
Susceptibility Test Methods
When available, the clinical microbiology laboratory should
provide the result of in vitro susceptibility test results for antimicrobial
drugs used in resident hospitals to the physician as periodic reports that
describe the susceptibility profile of nosocomial and community-acquired
pathogens. These reports should aid the physician in selecting an antibacterial
drug for treatment.
Dilution Techniques
Quantitative methods are used to determine antimicrobial
minimum inhibitory concentrations (MICs). These MICs provide estimates of the
susceptibility of bacteria to antimicrobial compounds. The MICs should be
determined using a standardized method (broth, agar, or microdilution)1,3.
The MIC values should be interpreted according to criteria provided in Table 1.
Diffusion Techniques
Quantitative methods that require measurement of zone
diameters also provide reproducible estimates of the susceptibility of bacteria
to antimicrobial compounds. The zone size provides an estimate of the
susceptibility of bacteria to antimicrobial compounds. The zone size should be
determined using a standardized test method2,3. This procedure uses
paper disks impregnated with 30 mcg cefaclor to test the susceptibility of
microorganisms to cefaclor. The disc diffusion interpretive criteria are
provided in Table 1.
Table 1: Susceptibility Test Interpretive Criteria for
Cefaclor
Microorganisms1,2 |
Minimal Inhibitory Concentration (mcg/mL) |
Zone Diameter (mm) |
S |
I |
R |
S |
I |
R |
Streptococcus pneumoniae |
≤ 1 |
2 |
≥ 4 |
- |
- |
- |
1 Susceptibility of staphylococci to cefaclor may be
deduced from testing only penicillin and either cefoxitin or oxacillin
2 Susceptibility of Streptococcus pyogenes to cefaclor may also be deduced from
testing penicillin |
A report of Susceptible indicates that antimicrobial is
likely to inhibit growth of the pathogen if the antimicrobial compound reaches
the concentrations at the site of infection necessary to inhibit growth of the
pathogen. A report of Intermediate indicates that the result should be
considered equivocal, and, if the microorganism is not fully susceptible to
alternative, clinically feasible drugs, the test should be repeated. This
category implies possible clinical applicability in body sites where the drug
is physiologically concentrated or in situations where a high dosage of drug
can be used. This category also provides a buffer zone that prevents small
uncontrolled technical factors from causing major discrepancies in
interpretation. A report of Resistant indicates that the antimicrobial is not
likely to inhibit growth of the pathogen if the antimicrobial compound reaches
the concentrations usually achievable at the infection site; other therapy
should be selected.
Quality Control
Standardized susceptibility test procedures require the
use of laboratory controls to monitor and ensure the accuracy and precision of
supplies and reagents used in the assay, and the techniques of the individuals
performing the test.1,2,3 Standard cefaclor powder should provide the following
range of MIC values noted in Table 2. For the diffusion technique using the 30
mcg disk the criteria in Table 2 should be achieved.
Table 2: Acceptable Quality Control Ranges for
Cefaclor
QC Strain |
Minimal Inhibitory Concentration (mcg/mL) |
Zone Diameter (mm) |
Escherichia coli ATCC 25922 |
1 - 4 |
23 - 27 |
Haemophilus influenzae ATCC 49766 |
1 - 4 |
25 - 31 |
Staphylococcus aureus ATCC 25923 |
-- |
27 - 31 |
Staphylococcus aureus ATCC 29213 |
1 - 4 |
-- |
Streptococcus pneumoniae ATCC 49619 |
1 - 4 |
24 - 32 |
REFERENCES
1. Clinical and Laboratory Standards Institute (CLSI). Methods
for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow
Aerobically; Approved Standard - Tenth Edition. CLSI document M07-A10, Clinical
and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne,
Pennsylvania 19087, USA, 2015.
2. Clinical and Laboratory Standards Institute (CLSI). Performance
Standards for Antimicrobial Disk Diffusion Susceptibility Tests; Approved
Standard - Twelfth Edition. CLSI document M02-A12, Clinical and Laboratory
Standards Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania
19087, USA, 2015.
3. Clinical and Laboratory Standards Institute (CLSI). Performance
Standards for Antimicrobial Susceptibility Testing; Twenty-fifth Informational
Supplement. CLSI document M100-S25. Clinical and Laboratory Standards
Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania 19087, USA,
2015.